Recently technology using near-field light has been proposed as a technology which allows recording and/or reproducing data on optical discs at high density.
As a focusing unit for generating near-field light, an optical system in which a focusing lens and a solid immersion lens (hereafter called “SIL”) are combined, is receiving attention lately. By the combination of a focusing lens and an SIL, a higher NA (Numerical Aperture) than that of a focusing lens can be realized. If a numerical aperture of the optical system is increased, the size of a light spot can be decreased, which allows higher density recording.
In the case of an optical system using an SIL, it is necessary for light, which comes out of the emission surface of the SIL, to enter the surface of the optical disc, so the distance between the SIL and the surface of the optical disc must be very short. In the case of an optical system used for an optical information recording and/or reproducing apparatus which records and/or reproduces information on a DVD or the like, the distance between the objective lens and the surface of the optical disc is approximately 1 mm, but in the case of an SIL, the distance between the emission surface of the SIL and the surface of the optical disc is approximately 100 nm or less. If the distance between the SIL and the surface of the optical disc fluctuates, the near-field light may not be obtained, or the SIL may collide with the optical disc. Hence a control to maintain the distance of the SIL and the surface of the optical disc to be constant is required.
In order to realize this control, a method called “gap servo” was proposed. This method is disclosed in Patent Literature 1. According to this method, light having a predetermined polarization component is detected out of the reflected light from the optical disc based on the near-field light. This light corresponds to light reflected from an area where the near-field light is generated, and is also called the “return light”. An actuator actively adjusts the positions of the focusing lens and the SIL in the optical axis direction, so that the quantity of light of this return light becomes constant. Thereby the distance (gap) between the SIL and the surface of the optical disc is controlled.
On the other hand, as a method for improving the recording density at the optical disc side, a multilayer disc, in which is a plurality of information layers are disposed in the optical axis direction, was proposed. Patent Literature 2 discloses an apparatus which records information on a multilayer disc by an optical system using SIL.
A tilt control, for actively controlling the tilt of the focusing unit so that the emission surface of an SIL and the surface of the disc become parallel with each other, was also proposed. This tilt control method will now be described with reference to FIG. 12A to FIG. 13B.
FIG. 12A to FIG. 13B are diagrams depicting the positional relationship between an optical disc 1 and SIL 13, a state of a return light spot 203 irradiated onto a detector (also called “photodetector”) 901, and a configuration of a tilt detection circuit.
FIG. 12A is a diagram depicting a positional relationship between the optical disc 1 and the SIL 13, and FIG. 12B is a diagram depicting the state of the return light spot 203 irradiated onto the detector 901 when the optical disc 1 and the SIL 13 are in the positional relationship shown in FIG. 12A. FIG. 13A is a diagram depicting a positional relationship of the optical disc 1 and the SIL 13, and FIG. 13B is a diagram depicting the state of the return light spot 203 irradiated onto the detector 901 when the optical disc 1 and the SIL 13 are in the positional relationship shown in FIG. 13A. The detector 901 consists of two sub-detectors 201 and 202.
In FIG. 12A, the distance a1 between the edge of the SIL 13 at the inner circumference side of the optical disc 1 and the surface of the optical disc 1 is longer than the distance a2 between the edge of the SIL 13 at the outer circumference side of the optical disc 1 and the surface of the optical disc 1. As FIG. 12A shows, the optical disc 1 is warped in a concave shape when viewed from the light entrance side, and if the emission surface of the SIL 13 and the surface of the optical disc 1 are not parallel, the distribution of the quantity of light of the return light spot 203 is not uniform, since the distance between the emission surface and the disc surface is not constant on the emission surface of the SIL 13.
In other words, according to the above mentioned principle of the gap servo, the quantity of light of the return light changes roughly in proportion to the distance between the SIL 13 and the optical disc 1. Therefore the quantity of light of the return light spot 203, at the side where the distance between the emission surface and the disc surface is short, is relatively low, and the quantity of light of the return light spot 203, at the side where the distance is long, is relatively high. The sub-detectors 201 and 202 detect the quantity of light of the return light spot 203 respectively, and convert it into electric signals. The differential circuit 401 outputs the difference of each electric signal converted by the sub-detectors 201 and 202, as a tilt detection signal. In the state of FIG. 12B, the differential circuit 401 outputs the tilt detection signal having a negative voltage value.
In FIG. 13A, the distance a3 between the edge of the SIL 13 at the inner circumference side of the optical disc 1 and the surface of the optical disc 1 is longer than the distance a4 between the edge of the SIL 13 at the outer circumference side of the optical disc 1 and the surface of the optical disc 1. As FIG. 13A shows, in the case of the optical disc 1 warped in a convex shape when viewed from the light entering side, the relationship of the quantity of light of the return light spot 203 at the side where the distance between the emission surface and the disc surface is shorter, and the quantity of light of the return light spot 203 at the side where the distance is longer, is the opposite of FIG. 12A and FIG. 12B. In the state of FIG. 13A, if the sub-detectors 201 and 202 detect the quantity of light of the return light spot 203 in the same manner as the case of FIG. 12B, the differential circuit 401 outputs the tilt detection signal having a positive voltage value.
The tilt control circuit controls an actuator holding the SIL 13, using a tilt detection signal, in a direction to cancel the tilt of the surface of the optical disc 1 and the SIL 13. The voltage value of the tilt detection signal becomes zero if the surface of the optical disc 1 and the emission surface of the SIL 13 become parallel. In this state, the aberration of the light spot irradiated onto the information layer of the optical disc 1 is the minimum, and the information can be accurately recorded or reproduced. The possibility of the SIL 13 and the optical disc 1 contacting can also be decreased.
In the above mentioned conventional method, however, the following problem exists in the case of recording or reproducing data on a multilayer disc.
Gap servo controls the distance between the disc surface and the SIL by detecting the return light, from an area where the near-field light is generated, using a detector. The size of the area where the near-field light is generated depends on the depth from the disc surface to the information layer. In other words, the area is largest when the information is recorded to or reproduced from an information layer most distant from the disc surface (this layer is called the “first information layer L0”), and the area is smallest when the information is recorded to or reproduced from an information layer closest to the disc surface (this layer is called the “Nth information layer L(n−1)”). This is because the focusing position in the optical axis direction changes depending on the information layers to be recorded or reproduced, so the spot size of the laser beam on the emission surface of the SIL changes. Therefore according to the change of the light spot size on the emission surface of the SIL, the spot size on the detector changes accordingly.
In the case of a single layer disc having only one information layer, the problem that occurs due to the change of the light spot size is not generated. This is because the distance from the disc surface to the focusing position of the laser beam is always constant, and the size of the area where the near-field light is generated is also constant, therefore it is sufficient if an appropriate detection lens is disposed on the optical path to reach the detector, so that the light spot size becomes the optimum on the detector.
In the case of a multilayer disc, however, the following problems occur. Now the problems on the multilayer disc will be described with reference to FIG. 14A to FIG. 15B.
FIG. 14A to FIG. 15B are diagrams depicting a positional relationship between the optical disc 1 and the SIL 13, a state of a return light spot 203 irradiated onto a detector 901 and a configuration of a tilt detection circuit when recording to or reproducing from the Nth information layer L(n−1). FIG. 14A to FIG. 15B show a case of the optical disc 1 and the emission surface of the SIL 13 which are parallel.
FIG. 14A is a diagram depicting a positional relationship between the optical disc 1 and the SIL 13, and FIG. 14B is a diagram depicting the state of the return light spot 203 irradiated onto the detector 901 when the optical disc 1 and the SIL 13 are in the positional relationship shown in FIG. 14A. FIG. 15A is a diagram depicting a positional relationship of the optical disc 1 and the SIL 13, and FIG. 15B is a diagram depicting the state of the return light spot 203 irradiated onto the detector 901 when the position of the detector 901 is shifted.
If a detection lens, with which the size of the return light spot 203 on the detector 901 becomes optimal, is used when information is recorded to or reproduced from the first information layer L0, the size of the return light spot 203 on the detector 901 becomes smaller than the size of the detector 901, as shown in FIG. 14B, when information is recorded to or reproduced from the Nth information layer L(n−1).
If the position of the detector 901 is shifted from the center of the light spot in this state, due to aging and temperature characteristics of the pickup, the state of the return light spot 203 irradiated onto the detector 901 becomes as shown in FIG. 15B. FIG. 15B shows an example when the position of the detector 901 is shifted Δm in a direction parallel with the dividing direction in the detector.
In this case, a difference is generated in the quantity of light which enters the two sub-detectors 201 and 202. As a result, offset voltage is generated in the tilt detection signal, although the optical disc 1 and the emission surface of the SIL 13 are parallel, and the tilt of the SIL 13 cannot be accurately controlled.
If a detection lens, with which the size of the return light spot 203 on the detector 901 becomes optimal, is used when information is recorded to or reproduced from the Nth information layer L(n−1), on the other hand, the size of the return light spot 203 on the detector 901 becomes relatively larger when information is recorded to or reproduced from the first information layer L0. At the moment, the diameter or length of one side of the detector used for a standard optical disc drive is about 100 μm. Larger detectors are also available on the market, but the frequency characteristics of detectors tend to drop as size increases. Therefore if the detector size is increased, a quick change in the quantity of light cannot be detected as electric signals, and therefore servo control cannot be performed with desired frequency characteristics.
Citation List
Patent Literature
Patent Literature 1: International publication No. 03/021583 pamphlet
Patent Literature 2: Japanese Patent Application Laid-Open No. 2004-46915